A power plant is disclosed in Patent Literature 1. This power plant is for driving drive wheels of a vehicle, and is equipped with an internal combustion engine and a rotating machine as power sources, and a stepless transmission and a planetary gear unit for transmitting power to the drive wheels. The planetary gear unit is a general single pinion type that has a sun gear, a ring gear, and a carrier, and the sun gear and the ring gear are connected to the drive wheels via a high clutch and a low clutch, respectively. The engine is connected to the carrier via a main shaft, and the main shaft is connected to an input pulley of the stepless transmission. Further, the rotating machine, the output pulley of the stepless transmission, and the sun gear are connected to each other via an auxiliary shaft.
The power plant constructed as above has a torque circulation mode mainly used during low-speed traveling as an operation mode thereof. In this torque circulation mode, the aforementioned high clutch is disengaged to thereby disconnect between the sun gear and the drive wheels, and the low clutch is engaged to connect between the ring gear and the drive wheels. This causes the torque of the engine to be transmitted to the carrier via the main shaft, and the torque of the rotating machine to be transmitted to the carrier via the auxiliary shaft, the stepless transmission, and the main shaft. Further, the torque transmitted to the carrier is distributed between the ring gear and the sun gear, and torque distributed to the ring gear is transmitted to the drive wheels. On the other hand, torque distributed to the sun gear is transmitted to the auxiliary shaft, and after being combined with the torque of the rotating machine, it is transmitted to the main shaft via the stepless transmission, and further after being combined with the torque of the engine, it is transmitted to the carrier. As described above, in the torque circulation mode, the power from the engine and that of the rotating machine are transmitted to the drive wheels while circulating through the stepless transmission and the planetary gear unit.
As described above, in the conventional power plant, during the torque circulation mode, the transmission of power from the engine and the rotating machine to the drive wheels is carried out necessarily via the planetary gear unit, and hence due to power transmission loss caused by mechanical mating of teeth of gears at the planetary gear unit, the driving efficiency of the power plant is reduced. Further, the power from the rotating machine is necessarily transmitted via the stepless transmission in addition to the planetary gear unit, and hence the driving efficiency of the power plant is further reduced due to power transmission loss at the stepless transmission.
Further, during the torque circulation mode, also when electric power is generated by the rotating machine using the power from the drive wheels, the transmission of power from the drive wheels to the rotating machine is performed necessarily via the planetary gear unit and the stepless transmission, and hence it is impossible to obtain a sufficient electric power-generating efficiency due to power transmission losses at the planetary gear unit and the stepless transmission. Further, during the torque circulation mode, when electric power is generated by the rotating machine using the power from the engine, the electric power generation is performed using power distributed to the sun gear via the carrier. Thus, in this case as well, the transmission of power from the engine to the rotating machine is performed necessarily via the planetary gear unit, and hence it is impossible to obtain a sufficient electric power-generating efficiency due to power transmission losses at the planetary gear unit.
Further, due to the use of the planetary gear unit, which is a complicated mechanism in which a plurality of gears and shafts are combined, the construction of the power plant is very complicated. Further, due to the backlash between teeth of gears of the planetary gear unit, it is impossible to control the rotational speed of the drive wheels in a fine-grained manner.
The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide a power plant which is capable of improving the driving efficiency and the electric power-generating efficiency thereof.
[Patent Literature 1]    Japanese Laid-Open Patent Publication (Kokai) No. 2004-175320.